docs: add docs for bson_concat(3)
docs: add docs for bson_concat(3)

ksuarz/libbson,ajdavis/libbson,rayyang2000/libbson,Machyne/libbson,mongodbinc-interns/libbson,mongodb/libbson,Convey-Compliance/libbson,mapr/libbson,remicollet/libbson,bjori/libbson,remicollet/libbson,Convey-Compliance/libbson,rayyang2000/libbson,paulmelnikow/libbson,mongodb/libbson,4Second2None/libbson_tmp,bjori/libbson,mongodbinc-interns/libbson,paulmelnikow/libbson,Machyne/libbson,remicollet/libbson,rcsanchez97/libbson,4Second2None/libbson_tmp,Machyne/libbson,mongodbinc-interns/libbson,derickr/libbson,paulmelnikow/libbson,ksuarz/libbson,rayyang2000/libbson,derickr/libbson,jqk6/libbson,rayyang2000/libbson,alexeyvo/libbson,derickr/libbson,4Second2None/libbson_tmp,remicollet/libbson,paulmelnikow/libbson,alexeyvo/libbson,paulmelnikow/libbson,4Second2None/libbson_tmp,Machyne/libbson,ajdavis/libbson,mongodbinc-interns/libbson,alexeyvo/libbson,jqk6/libbson,derickr/libbson,rcsanchez97/libbson,mongodb/libbson,jqk6/libbson,jqk6/libbson,bjori/libbson,mapr/libbson,rcsanchez97/libbson,ajdavis/libbson,ksuarz/libbson,Convey-Compliance/libbson,mapr/libbson